As is well known, the crystalline lens that controls the vision sometimes lessens its adjustment function or has its characteristics such as transparency of the lens itself deteriorated due to genetic or aging factors. This consequently causes problems such as refractive disorders including myopia, hyperopia and presbyopia, or even cataract and the like, which makes it hard to obtain effective vision. To deal with such situations, an aphakic intraocular lens (hereinafter called “intraocular lens” as appropriate) has been conventionally used that is set in place in the capsule to replace the crystalline lens after enucleation and removal thereof.
However, since the intraocular lenses that have been conventionally used are monofocal ones, the problem is that the eye function ends up with no focal adjustment, although the vision is restored after the eye surgery.
To cope with such a problem, an intraocular lens capable of generating multiple foci by adopting a diffraction lens described, for example, in Patent Document 1 and the like and making use of diffraction of light is proposed. The diffraction lens described in Patent Document 1 is provided with a diffraction grating with reliefs on the lens surface and made capable of forming two foci generated by the 0th order light and first order diffracted light. Therefore, a bifocal intraocular lens is available by assigning the foci by the 0th order light and first order diffracted light for far vision and near vision respectively.
However, it is increasingly recognized in recent years that a bifocal intraocular lens that adopts a conventional diffractive lens structure is not sufficient enough to improve vision. That is, in case of an intraocular lens that adopts the diffractive lens structure described in the above Patent Document 1, for example, the 0th order light and first order diffracted light are assigned for far vision and near vision respectively, which revealed a problem that allocating energy to the mid-section between the 0th order light and first order diffracted light becomes more difficult and the contrast in the intermediate vision range gets too low.
Under these circumstances, an intraocular lens that generates multiple foci by forming multiple areas with different reliefs on the lens in its radial direction is proposed in Patent Document 2, for example, in order to make it possible to generate more number of foci. However, the intraocular lens described in Patent Document 2 had a risk of failing to achieve the desired focal effect when the diameter of incident light beam varies in cases such as pupil shrinkage. In addition, even if the design of the lens is based on a consideration of physiological pupil diameter, it is not necessarily possible to insert an intraocular lens in the desired position relative to the pupil and keep it stable therein, which poses a risk of failing to achieve the desired focal effect due to the eccentricity of the lens.    Patent Document 1: U.S. Pat. No. 5,121,980    Patent Document 2: U.S. Pat. No. 7,188,949